Chemical Change & Rate of Reaction

Your Learning Journey

Start Here

1. Changes

→

Next Up

2. Reaction Rates

→

The "Why"

3. Collision Theory

→

In the Lab

4. Investigations

→

The Results

5. Interpreting Data

1. Physical & Chemical Changes

Physical Changes

+
  • No new chemical substances are produced.
  • These changes are often easy to reverse and relatively easy to separate.
  • Examples:
    • Changing state (e.g., ice melting to water).
    • Making a mixture.
    • Dissolving a solute in a solvent.

Chemical Changes

+
  • New chemical substances are formed with different properties.
  • Most are difficult to reverse.
  • Signs of a chemical change:
    • Colour change.
    • Temperature change.
    • Effervescence (fizzing).

Example: Colour Change

When orange-brown copper metal is added to a colourless solution of silver nitrate, the copper displaces the silver. This causes two colour changes:

  1. The solid changes from orange-brown copper to greyish-white silver.
  2. The solution changes from colourless to blue as copper nitrate is formed.
Displacement reaction of copper and silver nitrate showing colour changes.

2. Rates of Reaction & Factors

What is Rate of Reaction?

It's a measure of how fast the reactants are being used up or how fast the products are being formed.

Rate of reaction =
Amount of reactant used OR Amount of product formed Time

Factors Affecting the Rate

Temperature

Increasing temperature increases the rate of reaction. Particles gain more kinetic energy, move faster, and collide more frequently and with more energy.

Concentration / Pressure

Increasing concentration (for solutions) or pressure (for gases) increases the rate. There are more reactant particles in the same volume, leading to more frequent collisions.

Effect of concentration (more particles in same volume).
Effect of pressure (same particles in smaller volume).

Surface Area

Increasing the surface area (e.g., using a powder instead of a lump) increases the rate. More particles are exposed, leading to more frequent collisions.

Surface area effect (large cube vs. many small cubes).

Catalyst

A substance that increases the rate of reaction and remains chemically unchanged at the end. It provides an alternative reaction pathway with a lower activation energy.

Enzymes are biological catalysts. Iron is a catalyst in the Haber process.

Light

Some chemical reactions require light energy to happen. These are called photochemical reactions.

Increasing the light intensity increases the rate of these reactions. Examples include photosynthesis and the decomposition of silver halides.

3. Explaining Rates with Collision Theory

For a reaction to occur, reactant particles must collide. But not just any collision will do!

Condition 1: Energy

The colliding particles must have enough energy. This minimum energy is called the Activation Energy (Ea).

Condition 2: Orientation

The colliding particles must have the correct orientation (they must face each other in the right way).

The rate of reaction depends on the frequency of successful collisions.

How Factors Link to Collision Theory

Energy profile showing activation energy with and without a catalyst.

4. Investigating the Rate of a Reaction

We can measure the rate by tracking changes in gas volume, mass, or the formation of a precipitate.

Method 1: Gas Collection

Example Reaction: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

  • Using a gas syringe: Measure the volume of gas collected at regular time intervals.
    • Setup for gas collection with a gas syringe.
  • Using a measuring cylinder over water: Suitable for gases that don't dissolve in water.
    • Setup for gas collection by displacing water.

Method 2: Change in Mass

Example Reaction: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

  • Place the flask on a balance.
  • Plug the flask with cotton wool to let gas escape but prevent acid spray.
  • Record the decrease in mass at regular time intervals.
  • Note: This method is less accurate than using a gas syringe.
Setup for measuring mass loss with a balance and cotton wool.

Method 3: Precipitate / Colour Change

Example Reaction: Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

  • The solid sulfur (S) forms a precipitate.
  • Mix reactants in a flask placed over a paper with a cross ('X') drawn on it.
  • Look down through the solution.
  • Measure the time taken for the cross to disappear.
The

5. Interpreting Data from Rate Experiments

Understanding the Graph (Product vs. Time)

A typical graph of product formed vs. time.

How to Calculate the Rate at a Specific Point

To find the rate at a particular time, you need to find the gradient of the curve at that point.

  1. Draw a tangent to the curve at the required time. A tangent is a straight line that just touches the curve at that point.
  2. Construct a triangle on the tangent line to calculate the rise and run.
  3. Calculate the gradient using the formula:
Rate of reaction (or gradient) =
change in y change in x
Graph showing how to draw a tangent and calculate the gradient.

Worked Example

The reaction between Iodine and methanoic acid produces carbon dioxide gas. The rate can be found by plotting volume of CO2 vs. time.

I2(aq) + HCOOH(aq) → 2I-(aq) + 2H+(aq) + CO2(g)

To calculate the rate of reaction at 20 seconds:

  1. Draw a tangent to the curve at 20 seconds.
  2. From the example graph, the tangent passes through (0, 3) and (40, 27).
  3. Calculate the change in y: 27 - 3 = 24 cm3.
  4. Calculate the change in x: 40 - 0 = 40 s.
  5. Calculate the rate:
Rate =
24 cm3 40 s
= 0.6 cm3/s
Worked example graph from the PDF showing the tangent at 20 seconds with the calculation triangle.